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Hung Chung (Protek) 3502C Oscilloscope Repair Diagnosis

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Can Uysal

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Hi there, I just signed up for the forum to get some help, thanks in advance!

I saw a similar thread which led me here but it is not the exact situation here, so I'm opening a new thread for this.

The problem is I recently bought the oscilloscope in the title, It's working fine, but only channel 2 is working. I want to use xy function now, so I need to repair channel 1.

I'm currently a licence senior student on Electronics, but I lack the skills to diagnose the broken circuit part, and this is huge for me.

Can you fellow colleagues lead me to the solution? Here is a link for the schematics I found on subject.



New Member
Hi there, welcome to the forum.
Just to clarify, what exactly do you mean by "Broken"? If you set the scope to display channel 1 only, do you get any trace at all? If so, can the trace be translated as normal by adjusting the position dials?
Have you worked with HV before? If not be sure to keep extra safe.
I'm sure you already know this, but for completeness, obviously don't touch anything if the scope's plugged in, and make sure to discharge caps via a 1MΩ resistor to the chassis after you turn them off. The HV caps (specifically, C422 and C423 and anything near them) should be discharged via a resistor (ideally, use 3 330k resistors in series to deal with carbon resistors' low breakdown voltage) to avoid arc damage.

For easy reference, here is the relevant part of the diagram, GIMPed into one image:

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Can Uysal

New Member
Thanks for your reply Spurosity, and sorry for the late response. I figured out the problem, which is nothing at all.

I frustrated with a striped screw as I was opening the button pannel, then I left it out to calm down a bit. And then it hit me, I read somewhere in the forum to clean the switches and such, so I used some good old WD40 on the channel switches.

While I was pushing it up and down, I noticed something. The position knob was way up the screen! I returned it to normal and voila, chanel A was there. This might look dumb, but I had no idea how to use this scope when I got it, and the first thing I was able to work was channel B, and I thought channel A was 'broken' within all that fuss.

Anyways, It's working well, I ordered two p6100 100mhz probes for that glorious achievement, happy as hell, and I'm thankful for your help!

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